The present invention relates to a method of forming layers of getter material on glass parts.
Various industrially manufactured products require, for their correct operation, maintaining vacuum or the composition of a predefined gaseous atmosphere within an enclosure defined by tightly sealed glass walls. As examples of industrial products having glass walls inside which vacuum is required, cathode tubes, field emission displays (known as FEDs), IR sensors, or some types of lamps may be cited, whereas examples of industrial products requiring the presence of a controlled atmosphere are some types of lamps or plasma displays.
The desired condition of vacuum or gaseous composition is obtained through evacuation operations of the enclosure, possibly followed by its filling with the desired gaseous atmosphere and by the subsequent airtight sealing of the enclosure, generally by means of welding operations of or between the glass walls defining the same.
However, there are phenomena that tend to modify the above-mentioned desired condition over time, thus degrading the functional characteristics of the industrial product. A first phenomenon is the gas release from the same glass forming the walls of the sealed enclosure or from the materials forming portions of the industrial product being present therein (phenomenon known in the field as “outgassing”), particularly in case the space is evacuated. A second possible phenomenon is the permeation of some gases, hydrogen in particular, through the walls. Finally, another possible source of alteration of vacuum or desired gaseous atmosphere occurs during the sealing of the enclosure, which requires melting operations of portions of the walls or of a vitreous paste placed between two portions of the walls. This operation is carried out at relatively high temperatures (some hundreds of degrees Celsius) and can result in intense gas releases. The gases more commonly contributing to these degradation phenomena are hydrogen, oxygen, water vapor, carbon oxides, and some small size hydrocarbons.
In order to overcome such drawbacks it is known to employ a getter material inside the sealed enclosure. Getter materials have the property of being able to fix molecules of the above-mentioned gases by means of physical sorption phenomena or actual chemical reactions. Getter materials of the second type are preferable, because the chemical reactions they give rise to are not reversible depending on temperature, and thus the sorbed gases are no longer released, not even upon intense heating of the industrial product. In the rest of the text and in the claims “getter materials” will refer to the latter ones, which will also be simply indicated as getters. Getter materials useful for the purposes of the invention are some metals as titanium, zirconium, niobium or tantalum; or metal alloys, generally based on zirconium or titanium and further comprising one or more transition metals, Rare Earths or aluminum. Interesting getter alloys for the present invention are for example an alloy of weight composition Zr 84%-Al 16%, disclosed in U.S. Pat. No. 3,203,901; an alloy of weight composition Zr 76.5%-Fe 23.5%, disclosed in U.S. Pat. No. 4,306,887; an alloy of weight composition Zr 70%-Fe 5.4%-V 24.6%, disclosed in U.S. Pat. No. 4,312,669; and an alloy of weight composition Zr 80%-Co 15%-A 5% (where A indicates one or more Rare Earth metals) disclosed in U.S. Pat. No. 5,961,750. The four above-mentioned patents are in the name of applicant (Saes Getters S.p.A.), and the four cited alloys are sold by the applicant under the trademarks St 101, St 198, St 707, and St 787, respectively.
In some industrial products, the space for inserting the getter material is extremely reduced. This is the case, for example, of FEDs or plasma displays, where the inner space has a thickness of a few hundreds of microns. In these conditions it is not possible to employ common getter devices, formed of powders of the material inserted in a three-dimensional housing.
U.S. Pat. Nos. 6,042,443 and 6,472,819 disclose getter systems to be respectively inserted into a FED and a plasma display. However, the systems of these patents have the drawback of requiring a metal support for getter powders, which reduces the available thickness for the getter material. Moreover, these systems are not suitable in the case of industrial products with even lower thicknesses of the inner space, as for example microbolometers, i.e., miniaturized IR sensors with a glass window, produced through techniques derived from the semiconductor industry: a microbolometer and the process for its manufacturing are described, for example, in U.S. Pat. No. 6,252,229.
European published patent application EP 1,513,183 A1 discloses an alternative method of introducing a getter device into an industrial product having glass walls, the method comprising: forming (e.g. by compression in a mold) a flat getter body with desired lateral dimensions; laying the getter body onto the desired part of the glass wall; heating the getter body, or at least the portion directly contacting the glass up to the melting temperature of the getter, e.g. by laser radiation, exploiting the fact that glass is transparent to radiation (and thereby is not heated by it) while the getter material sorbs the energy of the same radiation, thus indirectly obtaining also the melting of the portion of the glass wall adjacent to the getter; finally, letting the whole solidify, obtaining the welding of the getter on the glass wall. This method is certainly new with respect to those previously known, but still suffers from some drawbacks. Firstly, the melting of getter materials requires high temperatures, in excess of 1000° C., which, mainly in the case of a fast heating and/or following cooling, may result in high thermal stresses in the glass wall and its breaking. The melting and subsequent solidification has also the drawback that portions of molten material could move laterally on the wall, thus coming into contact with undesired functional areas or portions inside the industrial product. Secondly, as the sorbing properties of getter materials depend on their exposed surface, these are generally used in the form of powders (possibly mechanically compacted) in order to provide a surface-area/volume ratio as high as possible. In the method of the cited patent application, the getter material is caused to melt and then re-solidify, and at the end of this process the material looks like a compact body of a reduced surface area and thus of poor sorbing properties. In addition, the method requires anyway the pre-forming of a self-standing getter body starting from powders. In order to have sufficient mechanical characteristics for being handled until it is placed in contact with the glass, the getter body must anyway have a size (and, particularly, a thickness) not lower than some hundreds of microns. This may cause problems in accommodating the getter body in the final industrial product in terms of thickness, particularly in case of industrial products of small size. Finally, this method still requires positioning the getter body on a glass part, particularly in the case of miniaturized industrial products wherein the lateral size of the space available for the getter may be very small, e.g. few tens of microns, the positioning precision may be critical, and in this case possible lateral movements of the molten material could be particularly dangerous.